1. Field of the Invention
The present invention relates to elements and devices configured to generate and detect terahertz waves, methods of generating and detecting terahertz waves, and a measuring device using a terahertz time-domain spectroscopic system.
2. Description of the Related Art
Terahertz (THz) waves are electromagnetic waves containing components in any frequency band from approximately 0.03 to 30 THz. Many characteristic absorptions originating from structures and states of various substances such as biomolecules are present in such a frequency band. By using the characteristic, an inspection technique that analyzes or identifies substances in a non-destructive manner has been developed. Also, application to a safe imaging technique by using terahertz waves instead of X-rays and application to a high-speed communication technique have been proposed. A method of generating terahertz waves includes using a nonlinear optical crystal to generate second order nonlinear optical effects (difference frequency generation). Representative nonlinear optical crystals include LiNbOx (Lithium Niobate or simply “LN”), LiTaOx, NbTaOx, KTP, DAST, ZnTe, GaSe, and the like. Generation of terahertz waves uses the second-order nonlinear properties of these crystals, upon which two laser beams with a frequency difference are incident. Specifically, in nonlinear crystal materials, difference frequency generation (DFG) can occur where two laser beams generate another beam with the difference of the optical frequencies of the two laser beams. Also, generation of single-color terahertz waves through an optical parametric process, and an optical rectification method by irradiation of femtosecond pulsed laser beams have been known.
As a process of generating terahertz waves from such a nonlinear optical crystal, electrooptic Cerenkov radiation has received attention lately. This phenomenon occurs if a propagation group velocity of laser beams propagating through the nonlinear optical crystal is higher than a propagation phase velocity of generated terahertz waves. In such a situation, the terahertz waves are radiated in a conical form within the nonlinear optical crystal like shock waves. This is a method of generating terahertz waves by excitation of progressive waves. Hence, by matching phases of terahertz waves generated from different wave sources in a radiation direction, the intensity of terahertz waves can be increased. For example, there is a report that, when a femtosecond laser beam with its wavefront inclined is incident on LN, phase matching is provided in a radiation direction of terahertz waves. See, Hebling et at., “Generation of high-power terahertz pulses by tilted-pulse-front excitation and their application possibilities,” J. Opt. Soc. Am. B, vol. 25, pp. B6-B19, (2008). (Hereinafter, referred to as document 1).
In the method described in document 1, to satisfy a phase matching condition, the wavefront of the laser beam is optically inclined and is aligned with the radiation direction of the terahertz waves. However, alignment is difficult for an optical system that adjusts the shape of the wavefront of the light beam. Hence, a system utilizing this method can be cumbersome and complicated. Also, in the method of the document 1, a nonlinear crystal bulk is used. Such a nonlinear crystal has a large loss for the terahertz waves. Hence, it is difficult to radiate terahertz waves with high output radiation.